As pointed out by Fabian and Schroeder, one has to install/trust the SSL certificate from the proxy itself (for example here is how it's done with Burp, a local proxy like Charles).
As per documentation, install the Charles certificate:
Visit https://chls.pro/ssl in Firefox. You will be presented with a certificate import dialog. Tick the option "Trust this CA to identify websites" and complete the import.
In Firefox you can either accept a prompted certificate when visiting an untrusted website (like your local proxy https://chls.pro/ssl) or import it manually via a (.PEM) file, like here:
In Charles go to the Help menu and choose "SSL Proxying > Install Charles Root Certificate".
In Firefox you can go to Preferences menu, then type certificate in the search box, click View certificate... then Your Certificates, then Import...
It can depend on your platform how you import certificates (e.g. Windows "installs" them).
To isolate the issue, are you able to first intercept HTTP traffic?